If you lack the knowledge of the Godhead and who Jesus is it is difficult to say you are a protestant. I is also difficult to be anything else but a cult.
If you define anything other than “Catholic” as Protestant, then I guess they are. However JWs see themselves as separate from any part of what they label as “Christendom.”
My sister is a JW and would rile at being called a Protestant. I also think that over time definitions can change to mean something different than originally intended. I see Protestants as a group of Christians other than Chatolic who embrace a similar Theology regarding the nature of God, scripture, etc. Since JWs do not define God or Christ in the same way and have rewritten the Bible to suit their beliefs, I would not consider them a part of mainstream Protestant Christianity.